Syncytia formation, a hallmark of a productive FFV infection, was absent in all cultures transfected with recombinant provirus, except pCF-Env-HIV-2F5, pCF-Env-HIV-4E10N, pCF-Env-HIV-4E10D, pCF-Gag-Ova and pCF-Gag-HA, correlating with particle release (S1C Fig). Here, ERVW-1 is linked to infection.